Arterial line insertion

Procedure

A small cannula in an artery, usually at the wrist, shows the blood pressure beat by beat and lets blood be drawn without new needles.

Updated Nov 13, 2026

What it is

An arterial line is a short cannula placed in a peripheral artery and connected to a fluid-filled, pressurised tubing system and a transducer, which turns each pulse into a continuous waveform and numbers on the monitor. It gives beat-to-beat blood pressure in unstable patients, in whom cuff readings are slow and inaccurate, and allows repeated blood gases without repeated punctures. The radial artery is the usual first choice: it is superficial, easy to compress and backed by the ulnar artery through the palmar arches; many units check this collateral flow first with the modified Allen test, while accepting that the test predicts ischaemia poorly. The femoral, brachial, axillary and dorsalis pedis arteries are alternatives. The cannula can go in directly over a needle, by a Seldinger technique with a separate or integrated wire, or by deliberately passing through the artery and withdrawing into it. A reading is only as good as the system: the transducer must be levelled at the phlebostatic axis (fourth intercostal space, mid-axillary line), zeroed to air, and the waveform checked for damping with a fast-flush test.

Indications

Shock or unstable blood pressure, especially on vasopressors or inotropes.
Frequent arterial blood gases or blood tests, for example in ventilated patients.
Major surgery with expected large blood loss or swings in pressure, or deliberate control of blood pressure.
Hypertensive emergencies treated with titrated intravenous drugs.
Patients in whom a cuff cannot read (severe obesity, burns, arrhythmias).

Contraindications and precautions

Poor collateral flow or an abnormal Allen test on that side: choose the other side or another artery.
Infection, burns or a vascular graft or fistula on that limb: avoid it.
Peripheral arterial disease, Raynaud phenomenon or vasculitis: higher risk of ischaemia.
Coagulopathy: not absolute, but choose a compressible site and press longer if it fails.
Avoid end arteries without collateral flow (the brachial is used only with care).

Equipment

Arterial cannula (often 20 G for the radial) or an arterial kit with an integrated guidewire.
Pressurised flush system with saline (heparinised in some units), non-compliant tubing and transducer.
Monitor with an arterial pressure channel and cable.
Gloves, skin antiseptic, drape, local anaesthetic, and a towel roll to extend the wrist.
Suture or securing device, clear dressing and a red arterial label.

Step by step (and why)

Prime the transducer set without air bubbles before starting — bubbles damp the trace and can embolise.
Check and record collateral flow — the hand depends on the ulnar artery if the radial thromboses.
Extend the wrist over a towel and tape the hand — this brings the artery up and holds it straight.
Clean, drape and infiltrate local anaesthetic over the pulse — pain causes arterial spasm.
Enter at about 30 to 45 degrees towards the pulse, or under ultrasound — a shallow angle follows the artery.
On a flash, lower the angle and advance the catheter or wire off the needle — the needle tip sits ahead of the catheter.
Remove the needle, press proximally and connect the tubing — compression stops back-bleeding while connecting.
Secure, dress and label as arterial — a loose line bleeds fast and a clear label prevents drug injection.
Level, zero and run a fast-flush test — it shows whether the trace is correctly damped.

Troubleshooting the arterial trace

Finding	Likely cause	What to do
Flat, rounded trace, low systolic	Overdamping: air, clot, kink, loose connection	Check tubing, remove air, flush, check the wrist position
Tall spiky trace, high systolic	Underdamping: long or compliant tubing, resonance	Shorten the tubing, use the correct set; trust the mean
All readings shifted up or down	Transducer level or zero wrong	Level at the phlebostatic axis and zero again
No trace	Disconnection, clot, stopcock closed	Check the connections and the patient first

Complications

Thrombosis or spasm with distal ischaemia: check colour, warmth and capillary refill of the hand.
Haematoma, bleeding or exsanguination after disconnection.
Catheter-related infection, more often with femoral lines and long dwell times.
False aneurysm or arteriovenous fistula.
Accidental drug injection, and air or clot pushed back towards the brain by vigorous flushing.

Next step

Compare the line with a cuff reading after setup; if they disagree, check level, zero and damping before acting.
Re-level and re-zero whenever the patient or bed moves, and after each disconnection.
Check the hand every shift: pain, pallor or slow refill means removing the line and a senior review.
Remove the line as soon as it is not needed, then press firmly for several minutes and check the site.
Call a senior after two failed attempts at one site.

Test yourself

Think of the answer, then tap the question to see it.

Where is the transducer levelled for an arterial line?

At the phlebostatic axis, the fourth intercostal space in the mid-axillary line, roughly the level of the right atrium; then it is zeroed to air.

What does an overdamped arterial trace do to the readings?

It flattens and rounds the waveform, underestimating the systolic and overestimating the diastolic pressure; the mean is relatively preserved.

Why must an arterial line be clearly labelled?

Because drugs injected into an artery can cause severe distal ischaemia and gangrene; the line is labelled and never used for medicines.

Why is the radial artery preferred for an arterial line?

It is superficial, easy to compress and the hand has a second supply through the ulnar artery and palmar arches if the radial thromboses.

Add to my cards (4)

Elsewhere in DurgiPedia

Connections

In Pharmacology2

← All clinical skills

Educational content; it does not replace supervised hands-on training, your institution's protocols or a prescriber's order. A procedure is learnt with a teacher and done only once you are trained to do it.

Sources: Central Line Management (StatPearls (NCBI Bookshelf), 2026); Allen Test (StatPearls (NCBI Bookshelf), 2024); Anatomy, Shoulder and Upper Limb, Forearm Radial Artery (StatPearls (NCBI Bookshelf), 2025).

Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake

Representative model, for educational purposes only.